The alkaline liquors produced by the electrolysis of concentrated sodium chloride brines in diaphragm cells generally contain between 9 and 13% by weight of sodium hydroxide and between 14 and 17% by weight of sodium chloride. As they leave the electrolytic cells, these hot alkaline liquors are usually transferred by way of metallic conduits to storage vessels and/or evaporators where they are concentrated in respect of sodium hydroxide while precipitating sodium chloride.
In view of the highy corrosive character of hot alkaline-liquors of high sodium chloride content, and particularly of liquors saturated or supersaturated with sodium chloride, that are treated in the evaporators, these evaporators are generally made of corrosion-resistant metals or alloys, for example of nickel or stainless steel.
The use of corrosion-resistant metals or alloys severely burdens the cost of equipment handling alkaline liquors.
Even when made of metals or alloys of high quality, the evaporators corrode progressively nevertheless during use.
It has been proposed, in the publication "Werkstoffe und Korrosion", 22nd year, first installment, 1971, p. 22, to retard the corrosion of 18/8 steel in contact with aqueous solutions containing halides, by maintaining the steel at an anodic potential in order to passivate it and by incorporating a corrosion inhibitor in the solutions treated, for example sulphate or nitrate ions. This solution nevertheless further burdens both the cost of the plants and the operating expenses.
It has also been proposed, on pages 23-29 of the aforesaid publication, to treat sodium aluminate liquors devoid of halides in evaporators made of ordinary steel, anodically polarised.
It has however been thought, until now, that equipments made of ordinary steel would not be suitable for treating hot alkaline liquors, containing halides it has particularly been thought, until now, that it was impossible to use evaporators made of ordinary steel for concentrating alkaline liquors substantially saturated or supersaturated with sodium chloride and resulting from the electrolysis of brines in diaphragm cells.